The present invention relates generally to the field of manufacturing ophthalmic lenses, especially molded, hydrophilic contact lenses, and more specifically, to a high speed, automated contact lens molding system for automatically producing ophthalmic lenses.
The direct molding of hydrogel contact lenses is disclosed in U.S. Pat. No. 4,495,313 to Larsen, U.S. Pat. No. 4,565,348 to Larsen, U.S. Pat. No. 4,640,489 to Larsen et al., U.S. Pat. No. 4,680,336 to Larsen et al., U.S. Pat. No. 4,889,664 to Larsen et al., and U.S. Pat. No. 5,039,459 to Larsen et al., all of which are assigned to the assignee of the present invention. These references disclose a contact lens production process wherein each lens is formed by sandwiching a monomer between back curve (upper) and front curve (lower) mold sections carried in a 2xc3x974 mold array. The monomer is polymerized, thus forming a lens, which is then removed from the mold sections and further treated in a hydration bath and packaged for consumer use. Hydration and release from the mold of this type of lens is disclosed in U.S. Pat. No. 5,094,609 to Larsen, and U.S. Pat. No. 5,080,839 to Larsen, both of which are assigned to the assignee of the present invention.
At the present time, partially automated and semi-automated processes are used in the production of soft contact lenses, however, high production rates are not achievable, partly due to the strict process controls and tight tolerances necessary in the production of high quality contact lenses.
Typically, the molds for these lenses are formed, generally by injection molding, from a suitable thermoplastic, and the molds, usually in frames associating a number of such molds with support structure are shipped from a remote molding facility and stored for use in a production facility for manufacturing contact lens blanks.
It is known that the use of lens molds maintained under normal atmospheric conditions leads to inhibition of, and thus incomplete and non-homogenous curing of the reactive monomer composition at the surface of the lens, which in turn can adversely alter physical properties of the lens. This phenomenon has been traced to the presence of oxygen molecules in and on the lens mold surface, which is due to its inherent capability of the preferred polystyrene molding material to sorb quantities of oxygen. During molding, this oxygen can be released to the polymerization interface with the reactive monomer composition in amounts which exceed acceptable maximums as determined by empirical testing. More specifically, the oxygen copolymerizes rapidly with the reactive monomer but the polymerization chain thus formed is readily terminated, the result being a decrease in rate of monomer reaction, the kinetic chain length, and the polymer molecular weight. The criticality of oxygen level and the difficulty of implementing effective control protocols may be appreciated by recognizing that the level of oxygen at the reactive monomer/mold interface must be controlled to approximately 300 times less than the concentration of oxygen in air (3xc3x9710xe2x88x923 moles/liter).
This recognized problem has been addressed in the art by careful but time consuming and laborious preconditioning of the molds utilizing chambers evacuated to approximately 1 torr and maintained in this condition for a period of not less than 6-12 hours. Any interruption of the work cycle such as might be caused by a power interruption requires reinitiation of the conditioning treatment.
Even brief exposure of the molds to air after degassing, as in normal manufacturing handling is detrimental; it has been learned that even a one minute exposure to air results in sufficient absorption of oxygen to require 5 hours degassing to reacquire an acceptable condition. Accordingly, a degassing operation immediately proximate the manufacturing line, particularly for large volume transfers of molds with different exposure times was deemed impractical, and no real improvement over the present system.
The problem is complicated by the fact that the front and back curves of the juxtaposed mold sections exhibit different thicknesses, leading to potentially different exposure of the reactive monomer composition to oxygen across the surfaces of varying cross-sections which could result in distortion of the lens and degradation of its optical properties. Thus, the concentration distribution of oxygen in the respective mold sections or halves remains symmetrical for short degas times, but becomes progressively less symmetrical for longer degas times, and the anomaly can cause uneven cure and different properties between the front and rear surface. For example, the convex male mold may be degassed within about 2 hours, whereas the concave female mold may not be entirely degassed even after 10 hours.
The commercial demand for soft contact lenses has dictated the development of continuous or at least semicontinuous manufacturing lines. The criticality of manufacturing specifications in turn demands automated handling of the lens manufacturing operation.
Another problem specific to the production process used to produce contact lenses in accordance with the teachings of the foregoing patents is that the mold portions are surrounded by a flange, and the monomer or monomer mixture is supplied in excess to front mold curve prior to the mating of the mold halves. After the mold halves are placed together to define the mold cavity, the mold is weighted and the excess monomer or monomer mixture is expelled from the mold cavity into the space between the flanges. Upon polymerization, this excess monomer or monomer mixture forms a waste by product known in the art as a HEMA ring (when hydroxyethylmethacrylate monomer is used) which must be removed to avoid contaminating the production line or the packaged lenses.
In these prior art processes, corona discharge devices are at times utilized to create an adhesion zone on the underside of the back curve mold half, to thereby cause the HEMA ring to preferentially adhere to the back curve at the time the mold haves are separated.
The prior art process for separating the mold halves and removing the lens consists of preheating, heating, prying and removal. Hot air provides the heating, mechanical leverage the prying, and the removal of the HEMA ring is manual. Heating the mold by convection is not an efficient heat transfer technique. From the time a mold array enters the heating apparatus until the back curve mold half is completely removed requires on the order of one minute.
The present method for removing the lens is to apply heat to the back curve mold half by means of a heated air stream. The heating is done in two stages: a preheat stage and a heat/pry stage. In the heat/pry stage, the mold is clamped in place and pry fingers are inserted under one side of the back curve of the mold, and an upward pry force is applied during the heating cycle. When the required temperature has been reached, the back curve mold portion breaks free and one end is lifted by the pry finger and the mold half is removed. The remaining mold and lens is then removed from the heating and prying station, where remnants of the HEMA ring are removed manually. The temperature gradient achieved in the convection heating of the lens is relatively small, since the time it takes to heat the back curve mold half enables significant conductive heating of the lens, thereby decreasing the gradient, and making separation of the molds difficult. Adding more heat to the lens mold at separation only causes the back curve mold to soften and impair efficient mechanical removal. Finally, manual removal of the remnants of the HEMA ring is labor intensive and costly.
While the aforesaid production processes have some efficacy in the production of soft contact lenses they suffer a number of disadvantages which have hindered the development of a high speed automated production line. When mold frames are demolded in large batch processes, a power outage at the wrong time can effectively shut the entire production line down for many hours after restoration of power, while a batch of frames is degassed and readied for production. In the alternative, expensive control systems are required to protect partially degassed frames during a power outage.
Further, the use of large mold arrays can cause registration problems for precision automated machinery if the polystyrene frame is distorted in any way.
The invention involves the improved manufacture of lens blanks for soft contact lenses and more particularly to subsystem stations, operations, procedures and protocols implemented in a continuous or at least semi-continuous automated manufacturing line to provide high speed, high volume production with a reduced number of defective lenses or lenses of impaired physical or optical characteristics.
The invention includes a method implemented by associated apparatus according to a protocol to control oxygen levels at the interface between the lens mold blank and the reactive monomer composition within levels for reliable production of lenses of acceptable optical quality under optimum manufacturing conditions, thereby substantially reducing defect levels.
It is therefore an object of the present invention to greatly minimize the exposure of the monomer or monomer mixture to atmospheric conditions, particularly oxygen, and to reduce the amount of dissolved oxygen in the monomer or monomer mixture used to produce the lenses.
It is also an object of the present invention to incorporate a completely automated production line system for automatically transporting contact lens mold portions throughout the contact lens filling, precuring, polymerizing, and demolding stations in a fast, efficient and precise manner.
Another object of the present invention is to provide a high speed apparatus for removing fragile front and back curve mold halves from a mold in which those articles are made, and then transporting those halves to and depositing those halves in a high speed, automated manufacturing system, in a low O2 environment.
A further object of this invention is to transport polystyrene mold halves from a mold in which those halves are made, and into a low oxygen environment of an automated contact lens molding system, in less than 15 seconds.
These and other objectives are attained with an apparatus for removing and transporting the mold halves from a mold, in which they are molded in an essentially oxygen free environment and transferred to the automated production line by robotic apparatus generally comprising first, second, and third robots or assemblies. The first assembly removes the mold halves from the mold and transports them to a first location, the second assembly receives the mold halves from the first assembly and transports them to a second location, and the third assembly receives the mold articles from the second assembly and transports the articles to a third location on pallets for entry into the automated line, while protecting the optical surface and where required, flipping the curve, for most efficient down stream processing.
It is still another object of the present invention to incorporate in an automated contact lens production line facility, an automated pallet system wherein a carrier pallet is provided that can receive both front curve lens mold portions and back curve lens mold portions prior to the formation of a lens mold assembly.
Specifically, the contact lens pallet system comprises a pallet for carrying and protecting the optical surface of one or more contact lens mold assemblies throughout an automated contact lens production line, the pallet having one or more first recesses formed in a surface thereof for receiving an individual contact lens mold assembly, the contact lens mold assembly comprising a first front curve mold half and a complementary second back curve mold half.
It is an object of the present invention to provide an apparatus for filling and assembling mold halves for a contact lens which includes a first automated station for receiving a plurality of front curve contact lens mold halves, carried in a unique pallet carrier, which are then filled with a predetermined amount of a polymerizable monomer or monomer mixture. The apparatus also includes a second automated station which provides a coating of surfactant on a portion of the front curve lens mold part to provide for preferential adhesion of any excess hydrogel to a back curve mold part. The apparatus further includes a third automated station for sequentially receiving a plurality of back curve mold parts, removing the back curve mold parts from the carrier pallet, and then receiving and registering the back curve mold parts with a plurality of front curve mold parts which were previously filled with the polymerizable monomer or monomer mixture. A vacuum is first drawn about the mold parts, and then the back curve is assembled with the front curve and weighted or clamped to displace any excess monomer from the mold cavity and to firmly seat the back curve mold part against a parting edge formed on the front curve mold part. The assembly is accomplished under vacuum to speed the assembly of the mold and to avoid the formation of gas bubbles from any gasses that might otherwise be trapped between the mold parts at the time of mold assembly.
It is also an object of the present invention to provide an apparatus and a method for precuring a polymerizable monomer or monomer mixture to form a soft contact lens in a mold which enables a more uniform cure for the lens during the cure step, and which reduces xe2x80x9cpuddlingxe2x80x9d or cavitation of the lens from the mold during cure. The mold halves are transported from the mold filling and mold assembly station to a precure station, where they are clamped together under predetermined pressure for a predetermined period of time in a low oxygen environment. The second or convex mold half is thinner than the first or concave mold halves to enable mold compliance during cure as the monomer or monomer mixture is polymerized. The clamping pressure aligns flanges formed on the first and second mold half to ensure that the flanges are parallel and that the respective curves of the molds are aligned. The clamping pressure also seats the back curve mold half against an annular edge formed on the front mold half to essentially sever any excess monomer from the monomer contained within the mold cavity, thus ensuring the best possible lens edge quality.
After a predetermined clamping period, the monomer or monomer mixture is exposed to actinic radiation, such as a UV light source, to partially cure the monomer or monomer mixture to a gel state. After a second predetermined period of exposure under clamping pressure, the clamping action and the UV radiation are removed, and the partially precured gel like lens is then transported in the mold through an extended curing station for complete polymerization and cure.
It is also an object of the present invention to provide methods and apparatuses that can easily and repeatably separate the contact lens mold portions having a contact lens formed therebetween without damaging the lens.
It is a further object of the present invention to provide a method and apparatus for separating a back curve mold from a front curve mold wherein a significant temperature gradient is created between the back curve mold and the contact lens contained in a cavity formed between the two mold portions.
It is another object of the invention to create this temperature gradient without excessive environmental heating or waste of energy through the use of laser beams or high energy steam nozzles.
It is another object of the present invention to provide an automated means to mechanically and reliably pry the mold halves apart in a consistent and reliable manner to thereby enhance the production of defect free lenses, and minimize the tearing of the lens or the breakage of the lens mold parts.
It is another object of the present invention to provide a method of controlling which mold half the lens sticks to by controlling the temperature gradient and pressure applied to the assembled mold during lens demolding.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.
Aspects and preferred features of the contact lens manufacturing system in part described and claimed herein are detailed in copending and commonly assigned application Ser. No. 08/257,802 of Martin et al. for xe2x80x9cLow Oxygen Molding of Soft Contact Lensesxe2x80x9d; application Ser. No. 08/257,801 of Walker et al. for xe2x80x9cLaser Demolding Apparatus and Methodxe2x80x9d; application Ser. No. 08/257,786 abandoned in favor of Ser. No. 08/462,811, abandoned in favor of Ser. No. 08/729,711 now U.S. Pat. No. 5,744,357 of Wang et al. for xe2x80x9cContact Lens Production Line Pallet Systemxe2x80x9d; application Ser. No. 08/257,267 of Lust et al. for xe2x80x9cApparatus for Removing and Transporting Articles from Moldsxe2x80x9d; application Ser. No. 08/257,785 of Lust et al. for xe2x80x9cMold Halves and Molding Assembly for Making Contact Lensesxe2x80x9d; application Ser. No. 08/258,264 of Martin et al. for xe2x80x9cMethod and Apparatus for Contact Lens Mold Filling and Assemblyxe2x80x9d; application Ser. No. 08/258,265 of Kindt-Larsen et al. for xe2x80x9cMold Separation and Apparatus; application Ser. No. 08/257,792 of Martin et al. for xe2x80x9cMold Clamping and Precure of a Polymerizable Hydrogelxe2x80x9d; application Ser. No. 08/258,263 of Kindt-Larsen et al. for xe2x80x9cMethod and Apparatus for Applying a Surfactant to Mold Surfacesxe2x80x9d; application Ser. No. 08/257,799 of Martin et al. for xe2x80x9cUltraviolet Cycling Oven for Polymerization of Contact Lensesxe2x80x9d; application Ser. No. 08/258,557 of Martin et al. for xe2x80x9cAutomated Apparatus and Method for Preparing Contact Lenses for Inspection and Packagingxe2x80x9d; and U.S. Pat. No. 5,294,379 of Ross et al. for xe2x80x9cLaser Assisted Demolding of Ophthalmic Lensesxe2x80x9d, the disclosures of which are incorporated herein by reference.